Locates the full path to the script bin directory to allow the use of paths
relative to the bin directory.

This allows a user to setup a directory tree for some software with
directories <root>/bin and <root>/lib and then the above
example will allow the use of modules in the lib directory without knowing
where the software tree is installed.

If perl is invoked using the -e option or the perl script is read from
STDIN then FindBin sets both $Bin and $RealBin to the current directory.

$Bin - path to bin directory from where script was invoked
$Script - basename of script from which perl was invoked
$RealBin - $Bin with all links resolved
$RealScript - $Script with all links resolved